A. Field
The present teachings generally relate to methods and apparatuses for electrical wire, and more particularly to free air fire alarm cable.
B. Background
Fire safety cable (critical circuit cable) finds application in providing electrical power to equipment and systems that are required to function during a fire. These systems may include fire alarm controllers, fire suppression equipment, sprinkler pumps in high rise buildings or other environments. This equipment needs to operate for a sufficient period of time to allow the safe evacuation of people the location of the fire.
Fire performance cables are required to continue to operate and provide circuit integrity when they are subjected to fire. To meet some of the standards, cables must typically maintain electrical circuit integrity when heated to a specified temperature (e.g. 650, 750, 950, 1050° C.) in a prescribed way for a specified time (e.g. 15 minutes, 30 minutes, 60 minutes, 2 hours). In some cases the cables are subjected to regular mechanical shocks, before, during and after the heating stage. Often they are also subjected to water jet or spray, either in the latter stages of the heating cycle or after the heating stage in order to gauge their performance against other factors likely to be experienced during a fire.
These requirements for fire performance cables have been met previously by wrapping the conductor of the cable with tape made with glass fibers and treated with mica. Such tapes are wrapped around the conductor during production and then at least one insulative layer is subsequently applied. Upon being exposed to increasing temperatures, the outer insulative layers are degraded and fall away, but the glass fibers hold the mica in place.
In the past the electrical power was provided through the use of mineral insulated cable. More recently, new and improved wire insulation material has been introduced for the safety cable (critical circuit) application. Today, a material of choice for wire insulation is a silicone rubber that has been specially formulated to form a ceramic-like layer when heated to the temperatures that are present in a fire.
The wire construction for safety cable (CI—“circuit integrity”) is typically a copper conductor. Over the copper conductor is applied the ceramifiable silicon rubber insulation. A jacket material is applied over the silicone insulation to provide mechanical protection during installation. One safety cable (CI) requirement for this family of cables is a fire test where the cables are installed in a manufacturer's specified system, and then tested for functionality in a furnace that models petroleum-fueled fire. In one test protocol the furnace is programmed to subject the test samples to a temperature rise on ambient to 1010° C. over a period of 2 hours. During this test the cables are energized to the voltage appropriate to the cables specified application. One test used is UL 2196 for 2 hours. To meet the requirements of the UL2196 test, electrical functionality must be maintained throughout the 2 hours and the following simulated fire hose water spray test.
The UL2196 test method described in these requirements is intended to evaluate the fire resistive performance of electrical cables as measured by functionality during a period of fire exposure, and following exposure to a hose stream. To maintain the functionality of electrical cables during a fire exposure the cables are tested using a fire resistive barrier. The fire resistive barrier is the cable jacketing if the jacketing is designed to provide fire resistance. If the cable jacketing is not designed to provide fire resistance, the electrical cables are either placed within a fire resistive barrier or installed within an hourly rated fire resistive assembly. Fire resistive cables intended to be installed with a non-fire resistive barrier (such as conduit) are tested with the non-fire resistive barrier included as part of the test specimen. Otherwise fire resistive cables incorporating a fire resistive jacket are tested without any barrier. To demonstrate each cable's ability to function during the test, voltage and current are applied to the cable during the fire exposure portion of the test, and the electrical and visual performance of the cable is monitored. The cable is not energized during the hose spray, but it is visually inspected and electrically tested after the hose spray. The functionality during a fire exposure of non-fire resistive electrical cables which are intended for installation within fire harriers or for installation within hourly rated fire resistive assemblies is determined by tests conducted in accordance with the UL Outline of Investigation for Fire Tests for Electrical Circuit Protective Systems, Subject 1724. Two fire exposures are defined: a normal temperature rise fire and a rapid temperature rise fire. The normal temperature rise fire is intended to represent a fully developed interior building fire. The rapid temperature rise fire is intended to represent a hydrocarbon pool fire. Two hose stream exposures are defined: a normal impact hose stream and a low impact hose stream. The low impact hose stream is applied only to cable intended to contain the identifying suffix “CI” to identify it as CI cable in accordance with the Standard for Cables for Power-Limited Fire-Alan Circuits, UL 1424, and in accordance with the Standard for Cables for Non-Power-Limited Fire-Alarm Circuits, UL 1425. In addition to fire alarm cables referenced in UL 1424 and UL1425, power cables can also be approved fir critical circuit applications. These power cables must meet the performance requirements listed in UL 444. Type RHH, RHW2, RHW and others of this standard if able to pass UL2196 can be qualified for CI applications,
In addition to the UL 2196 test, the circuit integrity (CI) must also meet the electrical requirements for non-CI rated cable. One of the requirements for this family of cables is long term insulation resistance. For this test, a copper conductor, with only the silicone rubber used as insulation, is tested at the specified voltage while the cable is immersed in 90° C. water. The insulation resistance is monitored periodically. The decrease in resistance must level out at a value above the minimum required. One of the requirements is specified in UL 444. This compound can pass the requirements of UL 2196, but is marginal to unable to meet the requirements of UL 444 for insulation resistance long term in 90° C. water at rated voltage.
This UL44 test specifies the requirements for single-conductor and multiple-conductor thermoset-insulated wires and cables rated 600 V, 1000 V, 2000 V, and 5000 V, for use in accordance with the rules of the Canadian Electrical Code (CEC), Part 1, CSA C22.1, in Canada, Standard for Electrical Installations, NOM-001-SEDE, in Mexico, and the National Electrical Code (NEC), NFPA-70, in the United States of America.
Plenum cable is cable that is laid in the plenum spaces of buildings. Plenum spaces are the part of a building that can facilitate air circulation for heating and air conditioning systems, by providing pathways for either heated/conditioned or return airflows, usually at greater than atmospheric pressure. Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum. In the United States, plastics used in the construction of plenum cable are regulated under the National Fire Protection Association standard NFPA 90A: Standard for the Installation of Air Conditioning and Ventilating Systems. All materials intended for use on wire and cables to be placed in plenum spaces are designed to meet rigorous fire safety test standards in accordance with NFPA 262 and outlined in NFPA 90A.
Plenum cable is jacketed with a fire-retardant plastic jacket of either a low-smoke polyvinyl chloride (PVC) or a fluorinated ethylene polymer (FEP). Polyolefin formulations, specifically based on polyethylene compounding had been developed by at least two companies in the early to mid-1990s; however, these were never commercialized, and development efforts continue in these yet-untapped product potentials. Development efforts on a non-halogen plenum compound were announced in 2007 citing new flame-retardant synergist packages that may provide an answer for an yet-underdeveloped plenum cable market outside the United States.
Plenum spaces allow fire and smoke to travel quickly. By using plenum-rated cable, the levels of toxicity in the smoke would be lower since plenum cable is coated with a jacket that is typically made of flame-resistant material such as Teflon®. This special jacketing, makes the cable smoke less than regular PVC cable and the smoke that is emitted is less toxic.
The NFPA (National Fire Protection Agency) is the body in charge of setting the code requirements for protecting plenum air spaces (as well as other fire concerns) and the National Electric Code or NEC is the standard they provide for handling all cables including power, network and video cabling. In NEC Section 800 it describes the properties of cables used for network and AV cabling. Any Nationally Recognized Testing Laboratory (NRTL) can certify NEC compatibility. Underwriter Laboratories (UL) is the de facto standard for making sure that cables meet or exceed all of the required specifications.
When exposed to fire, copper conductors may melt. At first, there is blistering and distortion of the surface. The striations created on the surface of the conductor during manufacture become obliterated. The next stage is some flow of copper on the surface with some hanging drops forming. Further melting may allow flow with thin areas (i.e., necking and drops). In that circumstance, the surface of the conductor tends to become smooth. The resolidified copper forms globules. Globules caused by exposure to fire are irregular in shape and size. They are often tapered and may be pointed. There is no distinct line of demarcation between melted and unmelted surfaces. As the copper conductor nears its melting point, the conductor softens and expands. The rate of expansion can be greater than the conductors ability to yield and the conductor buckles. At this point, the conductor can burst out of the insulation, which can lead to failure.